1. Technical Field
The technical field is embedded capacitors, particularly capacitors having thin film dielectrics.
2. Related Art
The practice of embedding capacitors in printed wiring boards (PWB) allows for reduced circuit size and improved circuit performance. Capacitors are typically embedded in panels that are stacked and connected by interconnection circuitry, the stack of panels forming a printed wiring board.
Fired-on-foil thin-film capacitor technology is known. U.S. Pat. No. 7,029,971 to Borland et al. discloses a chemical solution deposition (CSD) fired-on-foil thin-film process in which fired-on-foil thin-film CSD capacitors are formed by first depositing a thin capacitor dielectric precursor material layer onto a metallic foil substrate, typically by spin coating. Several spin-coated layers may be utilized. The metallic foil substrate may be copper foil and typically may range in thickness between 12 and 36 microns. The deposited CSD thin-film capacitor dielectric material is subjected to a firing or annealing process to crystallize the dielectric and increase the grain growth and consequently the dielectric constant. The firing process may be conducted at high temperatures, such as 900° C., in a reduced oxygen atmosphere to avoid oxidation of the underlying metallic foil. After firing, the dielectric layer will generally be a homogenous ceramic layer and may have a thickness of approximately 0.6 microns.
A metallic electrode is next deposited over the fired-on-foil thin-film ceramic capacitor dielectric layer. The deposition method for the electrode can be any of a number of deposition methods. Sputtering is generally the preferred choice. After deposition of the electrode, the thin-film capacitor may exhibit a high capacitance density and other desirable properties.
Embedded ceramic capacitors are subject to requirements such as high capacitance density, acceptable breakdown voltage, low dielectric loss, and high reliability, for example.
A high capacitance density capacitor can be achieved by using a thin film and a high dielectric constant dielectric in the capacitor. A requirement for high reliability and good breakdown voltage is a high level of densification, typically close to 100% density wherein any porosity in the film is isolated. Firing a CSD dielectric deposit on copper foil restricts the shrinkage to the “z” or vertical dimension when sintering takes place and combined with the high level of refractoriness exhibited by high dielectric constant materials, achieving high levels of densification is extremely difficult. When fired at 900° C., CSD dielectrics having six or fewer layers totaling between 0.5 and 1.0 microns in fired thickness, typically achieve densification percentages of 60-80%, the rest of the dielectric being porosity, the majority of which is interconnected. In addition to the reliability concerns, since air has a dielectric constant of 1, such levels of porosity will result in a reduction of the dielectric constant.
Thus, a problem to be solved in present electronic circuitry is the production of a dense CSD dielectric of a fired-on-foil capacitor while maintaining other desirable properties, such as high capacitance density. Firing at higher temperatures may be one approach to achieving higher levels of densification but the firing temperature has to be lower than the melting point of the metallic foil. In the case of copper foil, the firing temperature has to be less than approximately 1050° C. Accordingly, one mechanism for solving the problem of making a dense CSD dielectric is the addition of an inorganic glass flux to the dielectric precursor material, which acts to lower the annealing temperature of the dielectric whereby the foil does not melt.